Thermocompression bonding is a technique involving the simultaneous application of a predetermined amount of heat and pressure at the interface between two mating articles so as to effect a fusion type bond therebetween. Thermocompression bonding is frequently employed in demanding electronic assembly applications, such as in bonding an array of miniaturized gold plated leads, generally formed out of a lead frame, to a corresponding array of respectively aligned gold plated pads, which comprise accessible extensions of a metallized circuit. Such circuits are often fabricated on a relatively fragile substrate, typically of ceramic material.
In such thermocompression bonding applications, it becomes readily apparent that both the temperature and pressure employed constitute critical operating parameters, because either an insufficient or an excessive amount of either of these parameters can lead to defective thermocompression bonds. In addition, an excessive amount of heat and/or pressure can damage the circuit or substrate. Other important variables that affect the quality of a plurality of simultaneously produced bonds relate, for example, to the dimensional stability, uniformity of heat and hardness exhibited by the bonding member at the requisite elevated temperature. The thickness, hardness and cleanliness of the materials to be joined also affect the degree of material deformation and/or metal-to-metal fusion that occurs at the bonding site(s).
In attempting to distribute the necessary heat and pressure uniformly to all of the leads of an array thereof to be thermocompression bonded to an aligned array of circuit pads, an internally heated, and retractably mounted elongated bonding member has normally been employed to effect such bonded connections simultaneously. Such a bonding member, which normally includes at least one downwardly projecting bonding rail, is generally referred to as a "thermode", and will be so defined hereinafter.
A major problem encountered heretofore in using such a thermode is the fact that when it is heated to the necessary operating temperature (typically in a range of 750.degree. C. to 950.degree. C.) to effect reliable lead-pad thermocompression bonds, the bonding rail has had a tendency to become slightly bowed in either a concave or convex direction. Such bowing has been caused primarily by one or both of the following: (1) the establishment of a non-uniform temperature gradient along the longitudinal length of the thermode, and/or (2) the mounting of the thermode in a manner that restricts the thermal expansion thereof in all directions while being heated to the requisite elevated temperature.
With respect to the first-mentioned factor, a non-uniform temperature gradient has been found to result in many cases from the use of a thermode having a pronounced non-symmetrical cross-section relative to the elongated heater cartridge (or cartridges) mounted therein. This results in the thermode having wall areas that may vary appreciably in thickness and, hence, in temperature when heated.
As for the second above-listed factor, it has been common practice heretofore to fixedly mount the thermode, such as by threaded fastening members, to an associated support plate or hanger assembly, often referred to as part of a bonding head. As a typical thermode can grow, due to thermal expansion, at least 0.015 inch in even its smallest dimension when heated to a requisite bonding temperature, it is seen that tremendous forces are exerted on the fastening members. The resulting restricted freedom of the thermode to grow in all directions directly produces dimensional instability (non-uniformity), which contributes in a material way to a non-linear, and in most cases, a bowed bonding rail. Another problem that arises when using thermode fastening members of the threaded type, in particular, is that regardless of the material out of which they are made, they tend to become "welded" in the tapped holes of the thermode and/or support structure over a short period of time. This is caused primarily by both the temperature-induced surface oxidation and the thermal expansion-induced stress imparted thereagainst), making it very difficult to thereafter remove the thermode for repair or replacement.
Thus, while the bonding rail of a thermode of the type in question can be initially (or periodically) precisely machined so as to have what would normally be thought of as an ideal flat or linear surface. This has often proven insufficient in regard to ensuring that all of a plurality of simultaneously effected thermocompression bonds are of satisfactory quality. An initially machined-flat thermode bonding rail likewise has provided no assurance against the possibility of the circuit substrate cracking due to excessive pressure exerted against one or more discrete regions therealong, as a result of the the bonding rail, in many cases, having actually acquired only a slightly bowed profile after being heated to the requisite bonding temperature.
The severity of the thermode bowing problem can be more fully appreciated when it is realized that in one typical and demanding electrical lead-metallized circuit pad thermocompression bonding operation, any non-linear deviation along the thermode bonding rail (or rails) as small as 0.0005" can adversely affect the quality of some of the bonded connections. Such deleterious connections most often occur along either the center or end regions of the bonding rail, which region(s) depending upon the direction of any bow that is produced in a given thermode if neither compensated for nor obviated in some manner.
There have been several techniques employed heretofore, with limited success, to compensate for any temperature-induced bow in the bonding rail of a thermode and, particularly, when the latter has been fixedly secured to an associated support structure. One such technique has involved trying to grind a bow in the bonding rail, when cold, that is the mirror image of the bow normally found therein when hot, so as to ideally result in a heated bonding rail that is perfectly flat. This has proved very difficult, if not impossible, to accomplish in practice, primarily because a temperature-induced bow in a fixedly secured thermode, and/or in one having a variable temperature gradient along its length normally does not exhibit an arcuate profile that ideally approximates a smoothly generated, mathematically-defined curvature.
An alternative prior technique has been to grind the bonding rail flat after the thermode has been heated to the desired operating temperature. This is not only a hazardous operation, but poses a number of problems. More specifically, it is necessary to use a sound detecting technique to determine when the grinding wheel actually contacts the hot bonding rail. It has also been found that the grinding wheel normally creates a significant burr on the heated rail, which must be removed after the thermode is cooled to room temperature. In addition to these problems, the resulting bonding rail surface is also disadvantageously considerably more porous (rougher) after having been ground flat while heated than after having been ground with a counter bow while unheated.
Another technique employed heretofore to minimize temperature-induced bow along the bonding rail of a thermode has been to loosely suspend the latter from two pairs of longitudinally spaced, and downwardly extending brackets which, in turn, are rigidly secured at their upper ends to an associated support member or hanger of the bonder. Considered more specifically, the thermode is mounted on such brackets through the use of pins, each of which projects outwardly from a given thermode sidewall and into an aligned oversized keyway of the adjacent bracket. The function of the bracket keyways, of course, is to allow for the thermal expansion of the thermode in all directions when heated. Concomittently, the spacing between each pair of brackets is chosen so as to accommodate the width dimension of the thermode in relatively close fitting relationship therewith, thus ensuring accurate horizontal angular alignment of the thermode rail with a plurality of underlying circuit leads and pads, for example, to be thermocompression bonded together.
While such a mounting arrangement does allow for the relatively unrestricted thermal expansion of the thermode when heated, such expansion in the width dimension often results in the sidewalls of the thermode firmly contacting the brackets in a spring-biased manner. This frictional engagement has often been sufficient to cause the thermode to "hang up" on the brackets and, thereby, acquire a skewed orientation. When this "hang-up" condition occurs, precise parallelism of the thermode bonding rail relative to an underlying circuit substrate cannot be achieved in a consistent and reliable manner.
Still another approach employed heretofore to maintain a thermode bonding rail linear and, thereby, establish continuous parallelism between the rail and an underlying circuit substrate, is disclosed in U.S. Pat. No. 4,284,466 of G. A. Chayka et al., assigned to the same assignee as the present invention. As disclosed in that patent, the bonding rail forms part of a replaceable thermode insert, or bonding tip. By mounting the major portion of the bonding tip within a dovetailed groove formed in the main body portion of the thermode, a wedging action imparted against the bonding tip when heated minimizes the thermal resistance across the interfaces therebetween. Such wedging action is further relied upon to maintain the bonding tip in continuous parallelism with an inner mating reference surface of the main body portion of the thermode, which surface is initially adjusted so as to be in parallelism with an underlying article involved in a given bonding operation, such as a circuit substrate. Such established bonding tip parallelism, of course, is directly dependent on the major body portion of the thermode, of much larger mass, remaining precisely uniform dimensionally as it grows, as a result of thermal expansion when heated. As previously noted, such dimensional uniformity is very difficult to achieve whenever a thermode, whether of one piece or of multiple piece construction, is rigidly mounted to an associated support structure.
There thus has been an urgent need for a simplified, reliable and inexpensive technique for mounting a thermode in a composite thermocompression bonder such that the bonding rail, even after repeated use over extended periods of time, will remain linear, within exceedingly close tolerances, along its length while at the requisite bonding temperature. Only in this way can there be assurance that all of a plurality of simultaneously established thermocompression bonds produced with a given thermode will be of uniform and consistent quality and, in the case of substrate-supported lead-pad bonded connections, assurance that no damage will occur to either the leads, mating pads or substrate.